The invention concerns a radial piston pump in accord with the generic concept of claim 1.
Radial piston pumps are, among other types of pumps, employed in motor vehicles, for lubrication medium pumping in internal combustion engines and transmissions. Further applications are found in pumps for hydraulic positioning members, steering joints, springs, couplings, stepless drives, automatic controlled transmissions, hydraulic driving aids, auxiliary drives, concrete mixers, and the like. These pumps characterize themselves as particularly adaptable in installations wherein high pressures are involved.
Classified as positive displacement pumps, these pumps transport the pressure medium, not in continuous flow, but non-uniformly in increment volumes per rotation of an eccentrically centered drive cam. The cyclically transported volumes bring about, both on the suction side as well as on the pressure side, variations in pressure, pressure oscillations, or pulsations. These pressure variances are superimposed on intake and output impulses, which, arise by the opening and closing of the transport chamber. The pulsations are especially great, if during the use of inlet or outlet valves of the coil spring type, application volumes with large pressure variations are suddenly connected. Further, intense pressure swings may occur, if the pressure in the system is high or the cylinder is only partially filled.
When the pressure in a cylinder attains such a pressure as to trigger the opening of the annular coil spring closure element, then this element rises from its seating in the area of the respective cylinder and the hydraulic medium, i.e. the hydraulic fluid, is forced into the plenum. If the pressure in the cylinder falls under the closure point of the coil spring valve, then the respective closure element is forced back onto said seat and thereby causes a loud hammer sound. This procedure repeats itself for each rotation of the drive cam, and as often as the number of the piston-cylinder units in the pump.
The said noise is just so much louder, the more dynamically the opening and closing process runs, that is, in accord with how great the ratio is between the pressure at opening and closing and further, in accord with the slope of the pressure increase curve from the instant of the opening. If these several values run very high, then the coil spring of the valve is lifted distant from its seat very quickly and subsequently strikes accordingly hard on said seat upon its return.
DE 43 38 641 A1 discloses a generic radial piston pump, which possesses a pump body with cylinders closed off by plugs. In the plugs are found pressure channels, which are covered by a slotted coil spring which encompasses the body of the pump. Upon the rotation of the drive cam, the pistons are set into a thrust movement and force the already drawn in hydraulic fluid through the said pressure channels into an annular plenum on the other side of the coil spring valve, and do this as soon as the pistons close the intake entry port. Upon the ejection of the hydraulic fluid, and after the closing of the intake port, pressure peaks occur in the cylinder, which again produce a hammer sound and so stress the bearing shell inserts between the respective pistons and the drive cam.
DE 42 41 825 A1 describes another radial piston pump, in which, by the purposeful incorporation of elasticity in the piston, that is, on the connection between the piston and a needle bearing, the pressure and force gradients are diminished and the noise intensity is alleviated. DE 43 36 673 C2 makes known a radial piston pump, in which an annular spring is installed which encompasses the drive cam. The said spring is installed on the sliding element (bearing shell) between the drive cam and the piston. The pistons thus springingly support themselves in this arrangement on the drive cam, so that, upon the beginning of the expulsion stroke, the pistons relax the start of thrust, so that the pressure peaks are diminished.
Thus the invention has the purpose of reducing the noise and ameliorating the bearing stress. This purpose is achieved, in accord with the invention, by the features of claim 1. Additional embodiments arise from the subordinate claims.
In accord with the invention, in the base of the pistons, respectively, a connection boring to the drive cam has been placed, which is governed by a high pressure relief valve. Should the pressure in the cylinder exceed the opening pressure of the relief valve, then this will open so that hydraulic fluid escapes, and thereby, pressure peaks are reduced. At the same time, the escaping hydraulic fluid from the said connection boring, creates an hydraulic fluid buffer on the underside of the piston base, between this and the drive cam, that is to say, between the piston base and the bearing shell on the cam. This fluid pressure buffer acts to dampen the noise.
In an advantageous manner, the said relief valve can be installed in a hollow interior of the piston, which is open toward the pressure port. The relief valve is inset in the enveloping valve body which covers the connection boring. This body possesses a valve boring with a valve seat, which coacts with a closure element, by which the valve boring in the direction of the hollow space is shut off from the plenum. If the motion of the closure element is directed radially, then, inertial forces act upon said element, which forces are dependent upon the acceleration of the piston.
In accord with one embodiment of the invention, the proposal is made, of controlling the relief valve by means of these acceleration forces which act upon the closure element. This would be done in such a manner, that the opening, i.e. the closing, of the relief valve is carried out at the correct point in time. The opening of the relief valve is done, in this way, preferably shortly after piston movement passes the lower dead point and this opening initiates the diminution of the pressure peak. Correspondingly, the closing of the relief valve is done advantageously shortly after the passing of the upper dead point of piston movement, whereupon the intake suction procedure begins.
In a further development in this matter, the proposal is made, that the closure element is to be of such a mass, that the acceleration forces in the range of the lower dead point exceed the pressure forces in the cylinder, and in an upper dead point, such forces are diminished. Thereby, the connection boring is closed during the inward thrust of the piston, so that the full inlet suction pressure is achieved, while during the outward thrust the pressure peaks in the cylinder exceed the acceleration forces which are acting on the closure element in the closing direction. When this occurs, the relief valve opens and said pressure peaks are diminished.
In addition to the acceleration forces, the closure can be loaded by a valve spring. By means of such a spring, the opening and closing behavior of the relief valve can be modified. If the closure element is so placed, that it moves in a direction transverse to the direction of piston thrust, then a spring is necessary since no perceptible inertia exists.
In order to modify the speed at which the pressure peaks are diminished, it is of advantage, to design the valve boring and/or the connection boring to act as an orifice. Since the action of an orifice changes with the viscosity and with the temperature of the hydraulic fluid, it is a practical matter to be able to change the cross section of the said orifice boring correspondingly with the temperature, so that over large temperature ranges pressure peaks are ameliorated in the same manner.
The valve body can be inset in an optional manner into the piston. For instance, the valve body can be screwed in, welded in, pressed in as force fit or the like. In an advantageous design, the valve body possesses an outward flared rim which lies on the base of the piston, and on which one end of a piston spring abuts. With the other end, the piston spring supports a plug, which separates the cylinder from the plenum. Thus, the valve body is fixed by said piston spring between the plug and the piston base.
Further advantages arise from the following description with the aid of the drawings. An embodiment of the invention is presented in the drawing. The description and the claims embrace a multitude of features in combination.
The expert in this given technology will, in practicality, observe the features in their individual state and combine them into further significant combinations.